The invention relates to gas velocity or flow monitors wherein the transmit time of an ultrasonic pulse between an ultrasonic sender and receiver changes according to the velocity of the gaseous medium flowing between the sender and receiver.
Typically, a pair of ultrasonic transducers is mounted at an oblique angle to gas flow in a duct and on a common ultrasonic axis. The transducers operate as both senders and receivers and transmit time measurements are made with and against the direction of flow so that factors affecting sonic velocity such as temperature, pressure, etc. are eliminated and, only the transmit times, when the ultrasonic pulses are transmitted in the direction of flow and the sonic transit time against the direction of flow, are measured to determine gas velocity. These are denoted transit time flow meters as opposed to ultrasonic measurements which are based on Doppler effect.
The object of the present invention is to provide an improved transit time gas flow meter which is more accurate, easier to install and of lower cost.
In particular, an object of the invention is to provide a transit time ultrasonic measuring system for measuring gas flow which may be calibrated for zero flow bias. Further, a feature of the invention is that the up stream and down stream ultrasonic transducer reflector assemblies incorporate acoustically reflective surfaces which are easily mounted in acoustic view of each other. The upstream and downstream transducer assemblies incorporate a rectangular housing which includes a mounting bracket for orienting a transducer, particularly a low-cost narrow beam electrostatic transducer of the Polaroid.RTM.-type which are pre-oriented relative to a Teflon.RTM. coated reflective surface and includes a reference wire target which is positioned at the edge of the flow path of the gas being monitored. While the reference targets could advantageously be located along the acoustic axis of the reflective surfaces and the transducers, it preferably is located at the edge of the respective reflectors so that it is out of the path of any side lobes of the transducer and, even though it may not be in the right place, since it is a known distance, the signal microprocessor can easily attend to the fact that it is not on the acoustic axis. The reference targets provide for increased accuracy in flow measurement and zero flow calibration of the instrument. High temperature sound absorbing foam is applied to all non-reflecting surfaces to eliminate ringing.